In a boiling water reactor, a wedge is used in a riser bracket to reduce fluid vibration of a jet pump that is one of recirculation devices used for adjusting a flow rate of reactor water. However, wear due to the fluid vibration may degrade the wedge. Particularly, increasing the flow rate of the reactor water is supposed in increasing an output of an existing nuclear power plant, and in this case, it is predicted that a flow rate of the jet pump is increased to increase vibration of the jet pump. Thus, in terms of ensuring safety of the reactor, a technique is required of monitoring vibration of the jet pump and degradation of the wedge from outside a reactor pressure vessel during operation of the reactor.
As such a vibration monitoring unit, as shown in Japanese Unexamined Patent Application Publication (Patent Laid-Open Publication) No. 11-125688 (JP-A-11-125688) (Patent Document 1), a vibration monitoring apparatus for a structure inside a reactor using ultrasonic wave (ultrasound) is known. Further, Japanese Unexamined Patent Application Publication (Patent Laid-Open Publication) No. 2004-361131 (JP-A-2004-361131) (Patent Document 2) discloses a technique of separating a plurality of reflected waves and measuring and monitoring vibration displacement with high accuracy when the plurality of reflected waves are measured in an overlapping manner depending on the shape of an object to be monitored in using the same technique.
A jet pump is a core internal structure (in-core structure) disposed in a reactor pressure vessel, and an optical measurement unit can not be used to evaluate vibration via reactor pressure vessel and reactor water. A mechanical measurement unit such as a strain gage can be used but is actually difficult to use due to its complexity because a measurement line needs to be routed inside and outside a pressure boundary. Thus, it may be suitable that the measuring and monitoring unit using ultrasonic wave disclosed in Patent Document 1 is used to measure and monitor vibration of the jet pump from outside the reactor pressure vessel.
Meanwhile, for vibration of a jet pump, the existence of vibration modes is confirmed other than a vibration mode with vibration in a radial direction of a reactor pressure vessel (direction perpendicular to a longitudinal direction of the jet pump) as a low-order eigenmode. However, in both of the Patent Documents, vibration only in the same direction as the radius direction of the reactor pressure vessel, which is an ultrasonic wave incident direction, can be measured (monitored). A component of the jet pump has a curved surface, and there is a possibility that ultrasonic wave reflected by the surface of the jet pump can not be suitably received.
Further, wear due to fluid vibration may degrade the wedge of the jet pump, and the wear state needs to be checked. Generally, the wear state can be checked only in a state where the reactor pressure vessel is opened in a routine inspection (examination) during operation stop of the reactor. Thus, when it is found that the wear of the wedge extremely develops and the wedge needs to be replaced, entire preparation period for wedge replacement correspond, as it is, an extension period of a routine inspection period.